= 1 - Removal of chromium compounds from Cr(VI)-containing aqueous phases Technical field of the invention The present invention relates to the processing of heavy metal-containing aqueous phases and in particular to the removal of chromium ions of the VI oxidation state (Cr(VD)) on their own or together with any chromium ions present of the III oxida- tion state (Cr(III)) from aqueous phases.
Prior art Numerous purification processes have been proposed for the removal of chromium compounds from wastewater, leachate and groundwater — including not only the leachates from waste heaps of ferrochrome producers but also landfills of spent chromium ore used for the production of chromium(VI) chemicals.
The chromium concentrations in these waters are generally low and are in the range from a few ppm to single-digit percent.
In the case of solutions with higher concentrations, many of the processes briefly outlined below reach their economic and technolog- ical limits.
Numerous processes are proposed for relatively low Cr(VI) concentrations in the range of about 100 ppm, for example the treatment of the Cr(VI)-contaminated wastewater with steel wool (A.
Ozer, H.
S.
Altundogan, M.
Erdem, F.
Tiimen, En- vironmental Pollution, Vol. 97, No. 1-2, pp. 107-112, 1997). However, the treat-
ment proposed in said document only results in a reduction of the Cr(VI) to dis- solved Cr(IIT), and therefore only fulfils part of the stated object.
In addition, mag- nesium metal is proposed for the reduction of the water-soluble Cr(VI) compounds, despite the expected high costs for the metal (G.
Lee, J.
Park, Geochimica et Cos- mochimica Acta 102 (2013) 162-174).
The use of the mineral pyrite (FeS2) is examined for use for wastewater treatment.
This process leads, in a first step, to Cr(IIT) which needs to be separated off in a further step (Yao-Tung Lina, Ching-Pao Huangb Separation and Purification
= 2 - Technology 63 (2008) 191-199.), however the effects found here are not very large in relation to the amount of pyrite used.
Organic waste products are also proposed for use for Cr(VI) reduction and partial adsorption, for example rice straw or else grape waste.
The latter is described in Rumi Chand, Kenji Narimura, Hidetaka Kawakita, Keisuke Ohto, Takanori Watari, Katsutoshi Inoue Journal of Hazardous Materials 163 (2009) 245-250. The litera- ture also discusses processes based on a reduction of the Cr(VI) with Fe(II) ions generated electrochemically at electrodes.
These processes have the advantage of precipitating the Cr(III) formed directly as hydroxide (N.
Kongsricharoern and C.
Polprasert Wat.
Sci.
Tech.
Vol. 31, No. 9, 109-117, 1995). According to the authors, this process can be used even at chromium concentrations of nearly 4000 ppm.
An overview of the various processes is given inter alia by the following literature: Kirubanandam Grace Pavithra, V.
Jaikumar*, P.
Senthil Kumar, P.
Sundar Rajan, Journal of Cleaner Production 228 (2019) 580-593, as well as in V.
Madhavi, A.
Vijay Bhaskar Reddy, K.
G.
Reddy, G.
Madhavi and T.
Naga Venkata Krishna Vara Prasad Research Journal of Recent Sciences Vol. 2(1), January (2013), 71-83. Some of the processes dealt with in said documents are discussed in more detail below.
One process that is often used industrially for the reduction of the chromium(VI) present in the solution is the addition of a soluble iron(II) salt, such as iron(II) sulfate.
The reaction proceeds quickly and reliably in a broad pH range.
After the end of the reaction, excess Fe?" is oxidized to Fe?” with air.
The pH is then in- creased.
Chromium and iron precipitate as voluminous hydroxides and can be sep- arated off with the aid of precipitation and filtration aids.
The process can be used across a wide concentration range of Cr(VI) and Cr(III). However, Fe?” has to be used in a significant excess, which has a negative influence on the costs of the reactants and the technical implementation (higher volumes in the filtration, iron residues in the installation) as well as on the disposal costs.
Furthermore, the stoi- chiometric ratio of iron to chromium is unfavourable since at least three moles of iron must be used for one mole of chromium, as a result of which the chromium hydroxides are diluted by large amounts of iron hydroxides.
Besides this, there are further technical problems, for instance the clarifying filtration of the wastewater and the voluminous filter product. The sludge obtained is generally disposed of in landfills in a cost-intensive manner. A further process used industrially is the reduction of chromium(VT) by SO? or by sulfites such as sodium metabisulfite in acids, generally at a pH of 2, with subse- guent processing steps. The reduction proceeds at low pHs, has been known for a long time and can easily be observed by monitoring the redox potential (Applica- tion Data Sheet ADS 3300-02/rev.B May 2008, Rosemount Analytical Inc., 2400 Barranca Parkway, Irvine, CA 92606 USA). A Cr(III) salt solution produced in this way can be used in tanneries. However, the processing of the Cr(IIT) solution ob- tained, whether by separating off the dry salt or by concentrating the solution to higher chromium contents, in each case by evaporating water, is problematic: com- plete evaporation, which is often practiced, is worthwhile only in the case of very high concentrations, such as those not to be expected in the abovementioned types of water, and is therefore uneconomical. A concentration of the solutions is con- ceivable, but is also associated with high costs. The precipitation of the Cr(ILl) after the reduction, which is likewise conceivable and also practiced, specifically with the aid of sodium hydroxide solution or preferably calcium hydroxide and filter aids or flocculants as chromium hydroxide (mixed with gypsum or sodium sulfate) is possible and is also performed industrially, but is a demanding filter task involv- ing preceding sedimentation and subseguent washing, associated with a high level of apparatus complexity and corresponding capital costs and large waste streams. A further option is to purify the solution obtained after reduction of the Cr(VT) to Cr(III) by way of a selective cation exchanger, as described for example in F. Gode,
E. Pehlivan Journal of Hazardous Materials B100 (2003) 231-243 orin S. Kocaoba and G. Akcin Adsorption Science & Technology Vol. 22 No. 5 2004 or in S. Renga- raj, Kyeong-Ho Yeon, Seung-Hyeon Moon, Journal of Hazardous Materials B87 (2001) 273-287. The inherent problem with this process is that it is economically viable only for very low Cr(VI) concentrations in the water to be treated, since the number of adsorption sites on the ion exchanger is relatively low. As a result, only a highly dilute solution is obtained after the regeneration of the ion exchanger.
= 4 - Solutions in the low single-digit percentage range in relation to chromium are to be expected, which in turn worsens the ratio of the volume purified to the volume to be disposed of. An economically viable use can be assumed at lower concentrations of significantly below 10 ppm. In addition, in the case of removal by ion exchang- ers, ions that may already be dissolved in the starting water, or that have been de- liberately added as a result of pH adjustment steps or as a result of the preceding reduction, such as sodium or, specifically when using anion exchangers for the di- rect removal of the chromate, ions such as sulfates or chlorides, very significantly disrupt the efficiency and thus the economic viability of the processes. The de- scribed purification of the water (without previous reduction) with ion exchangers is discussed in Serpil Edebali, Erol Pehlivan Chemical Engineering Journal 161 (2010) 161—166 and is also — at low concentrations — used industrially. The purification of Cr(VT)-containing wastewater exclusively using specific, non- commercially available activated carbons has been discussed but works slowly and incompletely at low pHs, and so industrial use in large-scale plants is not conceiv- able (Manuel Peres-Candela, Jose M. Martin-Martinez, Rose Torregrosa- Macia,
Wat. REs. Vol 29, No. 9, 1995, 2174-2180). CN 108 928 953 A discloses a process for removing chromium from wastewater comprising a step of increasing the pH, where chromium as Cr(IIl) is precipitated in the form of hydroxide. Said document further discloses the addition of activated carbon to this precipitate. Object of the invention: There was therefore a need for a process for removing chromium compounds from Cr(VI)-containing aqueous phases which avoids the disadvantages of the above- described prior art and can be performed over a wide range of Cr(VI) concentra- tions cost-effectively and with a moderate level of apparatus complexity. An ideal process would i. enable a cleaning performance that allows the aqueous phase depleted in chromium(VI) to be sent to a conventional water treatment plant,
it. in doing so use mainly environmentally-friendly chemicals and iti. provide the separated-off chromium in a form that allows further econom- ically viable utilization. The object is achieved by a process for removing chromium compounds from aque- ous phases according to Claim 1. The chromium compounds to be removed are chromium ions of the VI oxidation state (Cr(VI)) which are dissolved or present in colloidal form in the aqueous phase and which are intended to be removed from the aqueous phase on their own or together with any chromium ions of the III oxidation state (Cr(IID)) also present in the aqueous phase in dissolved or colloidal form. The aqueous phase is typically solutions or suspensions or dispersions having a content of water of more than 50% by weight, preferably more than 80% by weight and particularly preferably more than 90% by weight. In further preferred embodi- ments, said aqueous phase is Cr(VI)-containing groundwater, leachate from waste heaps or wastewater, preferably wastewater from electroplating processes or wastewater from tanneries. Step a) Activated carbon, typically in pulverulent or particulate form, is introduced into the aqueous phase provided for purification and stirred. At pHs of 7, the activated carbon is not capable of taking up relatively large amounts of chromium or of re- acting with chromium(V1). For this reason, this step involves adjusting the pH to a value of 1.8-3 with a suitable acid, for example a mineral acid such as sulfuric acid, or hydrochloric acid or an organic acid, preferably with sulfuric acid. The pH may be adjusted before, during and/or after addition of the activated carbon, as long as it is ensured that the pH of the aqueous phase has a value of 1.8 to 3 at the end of step a). Preferably, the pH is adjusted after addition of the activated carbon.
Especially at a high content of Cr(VT), it may be advantageous to already reduce some of the Cr(VI) to Cr(IIl) in step a), for which a dwell time of the mixture in step a) (measured after completed addition of activated carbon and adjustment of the pH to a value of 1.8-3) of more than 5 minutes, preferably more than 15 minutes and particularly preferably of 0.5 to 76 hours is advantageous.
At least temporary mixing is preferably provided during the dwell time.
The mixing may be effected for example by circulation, stirring, pumping and/or blowing in gases.
The pH be- gins to increase again as the ensuing reduction progresses and should be compen- sated for by adding more sulfuric acid.
This causes the Cr(VI) content to fall sig-
nificantly within a few hours (see Figure 1). Some of the chromium is also directly absorbed onto the carbon, presumably partially after a reduction.
In step a) the con- tent of Cr(VI) in the agueous phase is preferably reduced to below 1000 ppm, pref- erably below 500 ppm and particularly preferably below 100 ppm.
In principle, the reduction of the Cr(VT) at a constant pH with activated carbon is also possible down to approx. 1 ppm as long as the pH is constantly adjusted, how- ever the dwell time needed for this makes the process less efficient.
In a storage test with pH adjustment by way of a highly acidic ion exchanger in the H form, an acceptable Cr(VT) value was achieved only after six weeks.
The treatment with activated carbon makes it possible to reduce a large part of the Cr(VI) after a few hours.
Figure 1 shows the Cr(VT) reduction using the example of a solution having 3512 ppm of Cr(VI) as a function of time in an agueous Cr(VD/Cr(IIl) solution at a largely constant pH of 2.8. The amount of sulfuric acid to maintain the pH is plotted here on the left, and the Cr(VT) value in the solution after filtration is plotted on the right.
The change in the Cr(VT) value can be determined directly by analytical monitoring of the Cr(VT) or indirectly by the consumption of sulfuric acid, but not by a change in the redox potential by means of corresponding electrodes.
In this case, the po- tential measured with a redox potential electrode, when a solid is involved in the reaction, changes only as a result of the changes in pH, but not with the change in concentration of the Cr(VI) species.
The reason for this is that the reducing agent, the activated carbon, is insoluble in the water and therefore does not enter into the eguilibrium.
The reduction of some of the Cr(VI) that is achieved as a result of the dwell time in step a) saves on reducing agents in step b), however it is entirely optional and represents a preferred embodiment.
In the context of the invention, the activated carbon consists of over 80% carbon and typically has an inner (BET) surface area of at least 300 m?/g.
The iodine num- ber should be above 300 mg/g.
The activated carbon in the Cr(IIT) precipitate-acti- vated carbon mixtures typically has an iodine number of above 300 mg/g.
Particularly preferred activated carbons for the reduction of Cr(VT) to Cr(III) are
Norit® SAE Super, Norit® GL50W, Bentonorit® CA1 and Norit® Super SA DD, which are sold by Cabot Corporation.
Step b)
If a reduction of Cr(VI) with activated carbon is dispensed with, or if Cr(VI) has been reduced to Cr(III) by means of activated carbon sufficiently but not com- pletely, in a further step remaining Cr(VI) is reduced to Cr(III) with at least one reducing agent different from activated carbon.
The less Cr(VI) has been reduced in step a) with activated carbon, the more of the reducing agent different from ac-
tivated carbon is needed in step b). It is therefore a case of an economic decision depending on the costs of the activated carbon, of the reducing agent different therefrom, the costs for any extended reaction and stirring time of the solution in tanks and costs for sulfuric acid at the corresponding location.
The water-soluble reducing agent used may be one or more compounds from the group comprising S(IV) compounds, such as sulfites, bisulfites, metasulfites and sulfur dioxide, organic acids such as formic acid and ascorbic acid, sugar or other aldehydes, phosphites, metals such as magnesium and calcium, nitrogen compounds such as hydrazine and further compounds known to reduce Cr(VI) in agueous phase by those skilled in the art.
Sulfites, bisulfites, metasulfites and sulfur dioxide are preferably used, in particular sodium metabisulfite Na2S20s, sodium sulfite, sodium bisulfite or fixing salt (so- dium thiosulfate) Na2S203. Sodium metabisulfite is very particularly preferably used.
In step b), the reduction is preferably performed until the content of Cr(VI) is less than 1.0 ppm, preferably less than 500 ppb and particularly preferably below 300 ppb.
A preferred embodiment is as follows: if, during the dwell time in step a), the pH increases only very slowly and it can therefore be assumed that the reaction between activated carbon and Cr(VT) is virtually complete, water-soluble reducing agent is added — at a pH of 1.8 to 3, preferably 2.0 — as a solution or solid while the redox potential is monitored.
The redox potential should fall drastically with the addition and then remain constant at a significantly lower level despite further ad- dition of water-soluble reducing agent.
This reaction is comparatively quick, and the new redox equilibrium should have been established after a few minutes up to an hour.
The pH may need to be readjusted.
The measured redox potential is also a function of the pH, which must be kept constant.
Typical profiles of the reduction potential as a function of the addition of water-soluble reducing agent are shown in Figure 2, once without prior reduction with activated carbon and once after 5 h of dwell time in step a). Step c) If it can be assumed that the reduction of the Cr(VT) is sufficient (monitoring of the redox potential using a probe, possibly additional Cr(VI) determination), the pH is increased, while mixing, to a value of 7.0 to 9.0 in order to precipitate the Cr(III).
The Cr(IID) precipitate, which usually precipitates out very finely and is very diffi- cult to separate off, here forms with the activated carbon a chromium(III) precipi- tate-activated carbon mixture which is surprisingly easy to separate off.
The term *Cr(III) precipitate-activated carbon mixture” is intended to express that all precipitates of Cr(III) compounds on activated carbon that are obtainable in the process according to the invention by raising the pH in step c) are encompassed by said term.
Preferably, the pH is increased to a value of 8.0 to 9.0 in step c).
The mixing in step c) is effected for example by means of circulation, stirring, pumping and/or blowing in gases.
Step d)
The Cr(IIT) precipitate-activated carbon mixture may be separated from the liquid phase by way of example by filtering-off for example in a suction filter or a filter press, by centrifugation, sedimentation by means of gravity or filtration on filter candles.
The process according to the invention surprisingly results in a Cr(III)
precipitate-activated carbon mixture which can be filtered off or sedimented ex-
tremely easily in comparison with a Cr(IIT) precipitate which has not been precipi- tated in the presence of activated carbon under basic conditions.
Virtually all of the chromium is on the activated carbon.
For example, the filtration may be performed using a normal paper filter in a short time.
The optional use of a fine filter having a pore size below 1 um makes it possible to separate off even traces of the solid and thereby further reduce the total chromium content of the separated-off solution.
The agueous phase obtained in step d) typically has a total chromium content of below 2 ppm, preferably below 1 ppm and particularly preferably below 0.1 ppm.
The Cr(IID) precipitate-activated carbon mixture may optionally be dried.
Depend-
ing on the amount of activated carbon used, the chromium content is above 10% by weight, calculated as metal.
The filtered agueous phase is analysed with respect to
= 10 -
total chromium and after being released may be discharged into the wastewater system.
Of the four process steps of the present process, the first three may be performed in one single reaction vessel.
Significant advantages over the prior art are that the activated carbon can be used as an alternative and environmentally friendly reducing agent without, as discussed in the prior art, simultaneously binding the Cr(IIl) to the activated carbon, and that the activated carbon immediately absorbs the Cr(IIT) precipitate which precipitates out and thus converts it into a filterable form.
In addition, the activated carbon highly concentrates the chromium, this reducing both the volume of waste and the disposal costs.
A further advantage is that the Cr(IIT) precipitate-activated carbon mixture is a val- uable raw material for the ferrochrome industry, since it firstly has a high propor- tion of chromium and secondly, with the activated carbon, consists of a reducing agent, which is indispensable for ferrochrome production.
The present invention therefore also relates to Cr(IIT) precipitate-activated carbon mixtures according to Claim 11. The Cr(IIT) precipitate-activated carbon mixtures preferably have a content of chromium of 9% to 30% by weight, measured as Cr(0). The sedimentation rate of the chromium(III) precipitate-activated carbon mixtures is usually in the range from 0.05 to 0.2 cm/min.
Furthermore, the present invention relates to the use of Cr(IIT) precipitate-activated carbon mixtures for producing ferrochrome according to Claim 14.
Examples
Analysis: Determination of total Cr and Cr(VI)
The samples are analysed using colorimetric test devices and processes in accord- ance with ISO 5398-2. The measurement consists of two independent
= 11 - measurements, i.e. Cr(VI) before and Cr(VT) after an oxidation step of the sample. The oxidation step oxidizes all Cr metal and Cr(IIl) in the sample to Cr(VI). The difference between the Cr(VI) values before and after the oxidation step reflect the content of Cr(0) and Cr(III). Specimen Contaminated and filtered groundwater (origin: South Africa) having total Cr amounting to 1022 ppm and 799 ppm of Cr(VI) (yellow to orange in colour, low haze) was used as starting material in the tests described below. The pH of the filtered groundwater was 8. The solution and mixtures thereof were stirred during the tests. All experiments were performed at ambient temperature and pressure. Example A: Purification of the groundwater with activated carbon type A and re- ducing agent
0.5 g of activated carbon Norit SAE Super was added to 200 g of groundwater in a beaker equipped with a stirrer, a pH probe and a Mettler-Toledo ORP probe (redox potential probe reference electrode Ag/AgCl in 3 mol KCl) and the mixture was then adjusted to a pH of approx. 1.8 by adding sulfuric acid 50% (1.7 g). Subse- quently, a solution of Na2S20s5 10% in demineralized water was added dropwise, while the ORP (reference electrode Ag/AgCl in 3 mol KCI) and the pH were mon- itored closely. The pH slowly increased at the beginning of the metering and was then adjusted to 2 and kept at this pH by adding a few drops of sulfuric acid 50%. The dropwise addition of 10% strength Na?S2Os was stopped, after which the redox potential output by the probe was below 250 mV and no longer changed signifi- cantly by adding further solution. The highest drop in the redox potential (highest value in the first derivative) was determined after 5.1 g. The pH was then adjusted to 8.5 with a 50% strength aqueous NaOH solution
(1.4 €).
= 12 - The mixture was filtered through a filter paper, and the total Cr value and Cr(VT) of the filtrate was determined. The filtrate was clear and colourless. The filtrate had 0 ppm of Cr(VI) and 2 ppm of total Cr. Example B: Purification of the groundwater with activated carbon type B and re- ducing agent
0.5 g of activated carbon Organosorb 220-35 was added to 200 g of groundwater in a beaker equipped with a stirrer, a pH probe and a Mettler-Toledo ORP probe (re- dox potential probe) and the mixture was subsequently adjusted to a pH of approx.
1.8 by adding 50% strength sulfuric acid (1.7 g). Then, a solution of 10% strength Na2S205 in demineralized water was added dropwise, while the redox potential (reference electrode Ag/AgCl in 3 mol KCI) and the pH were monitored closely. The pH slowly increased at the beginning of the metering, was then adjusted to 2 and kept at this pH by adding a few drops of 50% strength sulfuric acid. The dropwise addition of Na2S20s 10% was stopped, after which the redox poten- tial determined by the probe was below 250 mV and no longer changed signifi- cantly by adding further solution. The pH was then adjusted to 8.5 with a 50% strength aqueous NaOH solution
(1.5 2). The mixture was filtered through a filter paper, and the total Cr value and Cr(VT) of the filtrate was determined. The filtrate was clear and colourless. The filtrate had 0 ppm of Cr(VI) and 3 ppm of total Cr. Example C: Purification of the groundwater with activated carbon type A, with a longer dwell time for the pre-reduction with activated carbon in order to save on reducing agent in comparison with Example A).
= 13 -
0.5 g of activated carbon Norit SAE Super was added to 200 g of groundwater in a beaker eguipped with a stirrer, a pH probe and a Mettler-Toledo ORP probe (redox potential probe) and the mixture was then successively adjusted to a pH of approx.
1.8 by adding sulfuric acid 50% (1.7 g). The mixture was stirred for 5 hours. Then, a solution of 10% strength NaxS20s in demineralized water was added drop- wise, while the ORP (reference electrode Ag/AgCl in 3 mol KCl) and the pH were monitored closely. The pH slowly increased at the beginning of the metering, was then adjusted to 2 and kept at this by adding a few drops of 50% strength sulfuric acid. The dropwise addition of Na2S20s 10% was stopped, after which the redox poten- tial output by the probe was below 250 mV and no longer changed significantly by adding further Na2S205 10% solution. The highest drop in the redox potential (high- est value in the first derivative) was determined after addition of 4.7 g of the solu- tion. The pH was then adjusted to 8.5 with a 50% strength agueous NaOH solution
(1.5 2). The mixture was filtered through a filter paper, and the Cr(VI) of the filtrate was determined. The filtrate was clear and colourless. The filtrate had 0.4 ppm of Cr(V1). Example D: Purification of the groundwater with activated carbon
439.6 g of activated carbon Norit GLSOW was added to 353.2 kg of filtered ground- water (pH = 6.65) with the analytical data as above and suspended. 1.701 kg of 50% strength sulfuric acid was subsequently added with stirring. The pH was 2.0; the redox potential (reference electrode Ag/AgCl in 3 mol KCI) was 605 mV.
= 14 -
6.0 kg of Na2S20s (20% strength solution) was added gradually with stirring, with 50% strength sulfuric acid (1.372 g) being added alternately in order to keep the pH at 2. The redox potential after complete addition was 236 mV; the pH was 2.03. The mixture was stirred for 20 minutes. Then, 2560 g of NaOH (50%) was added slowly; the pH was 8.58 after complete addition. After the stirrer was stopped, a black precipitate immediately settled out. The upper phase after the settling had a chromium total of 143 ng/kg. A sample of the liquid was filtered through a 200 nm syringe filter. The liquid had 54 pg/kg of total Cr. A sample of the upper phase after the settling was mixed with an ion exchange resin Lewatit TP107 (LANXESS). After a contact time of two hours, the liquid had a total chromium content of 11 pg/kg. Example E: Depletion of low-concentration solutions 53 mg of activated carbon Norit GLSOW was added to 2000 g of a solution of Cr(VD (5.11 mg/kg) and the pH was adjusted to 2.0 with sulfuric acid (50% strength). 10% strength Na2S20s solution was added dropwise until the redox potential (ref- erence electrode Ag/AgCl in 3 mol KCl) was <250 mV. The pH was kept constant by adding sulfuric acid. The pH was adjusted to 8.9 with 50% NaOH, the resulting black slurry was filtered through a filter paper and the filtrate was analysed with respect to total Cr, which was 1570 ug/l.
The filtrate was filtered with a 0.2 um syringe filter and analysed with respect to total Cr, which was 8 ug/l. Example F: Determination of the filter cake resistance In a 3-litre beaker, 5 g of activated carbon Organosorb 220-35 was added to 2000 g of groundwater. added in a beaker eguipped with a stirrer, a pH probe and a Mettler- Toledo ORP probe (redox potential probe) and the mixture was then adjusted to a pH of approx. 1.8 by adding sulfuric acid 50%. Then, a solution of Na2S20s 10% in demineralized water was added dropwise, while the ORP (reference electrode Ag/AgCl in 3 mol KCl) and the pH were kept constant by further addition. The dropwise addition of Na2S20s 10% was stopped, after which the redox poten- tial output by the probe was below 240 mV and no longer changed significantly by adding further Na2S205 10% solution. The pH was then adjusted to 8.51 with a 50% strength aqueous NaOH solution. The mixture was filtered through a filter paper on a suction filter (diameter 150 mm) with vacuum. The dried filter cake (height: 3 mm) was analysed: Cr 18% by weight. 38 minutes were needed for a volume of 2.1 litres. The cake resistance ac was approximately determined from the following formula, assuming a constant volume flow during the filtration: 1 dv Ap i 4 = aa + PR e da %
A. dt mh. rah) in which AF is the filter area (0.0176 m”), dV/dt is the volume flow
(9.21 x 107 m/s), Ap is the pressure difference of 1 bar (10° N/m?), m is the viscosity of water (1 N-s/m?), hx is the cake height of 0.003 m and Bm is the re- sistance of the filter medium, which was neglected.
This gives a cake resistance of less than 10'5 m”.